A recent flight test program by the National Aeronautics and Space Administration has demonstrated that a large reduction in fuel consumption by piston engine aricraft during flight operations can be safely achieved by utilizing extremely lean fuel-to-air mixtures. The fuel reduction is achieved by reducing the fuel-to-air ratio to a low level and concomitantly advancing the ignition timing. It was believed in the prior art that a practical implementation of such low fuel-to-air ratios and such advances in ignition timing required a sophisticated electronic computer to coordinate the fuel-to-air ratio, the advance in ignition timing, and the throttle position. It was also believed in the prior art that a continuous variation in the advance in ignition timing during the leaning operation was required in order to effectuate the desired reduction in fuel consumption.
However, a careful review of the National Aeronautics and Space Administration test data obtained in the ground running of a test aircraft engine prior to its flight test, lead the present inventor to the conclusion that the low fuel-to-air ratio may be achieved by a simple two-position controller which switches from a rich fuel-to-air ratio to a lean fuel-to-air ratio of approximately 0.05, or 75% of stoichiometric; and which switches the ignition timing from a normal 20.degree. before top dead center to approximately 35.degree. before top dead center. This conclusion that a two-position controller would be adequate was based on the fact that the minimum fuel consumption achieved by leaning at constant power always occurs at nearly the same fuel-to-air ratio for all engine operating conditions tested.
In order to achieve the desired low fuel-to-air ratio, however, and still avoid rough engine operation as the ratio is decreased, the ignition timing must be advanced. The data revealed that acceptable if not optimum results could be achieved with a constant ignition advance of 35.degree. before top dead center. If the ignition timing is not advanced, the fuel consumption will not decrease beyond a fuel-to-air ratio of 85% and, the engine will run rough and excessively hot at this point. However, leaning the fuel-to-air ration will yield minimum fuel consumption, and at the same time will provide smooth operation and acceptable cylinder head and exhaust temperatures.